Working with Offsets in Graphical Rating and Shift Application Tool

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Detailed Description

This video show how use offsets when developing ratings in GRSAT.


Date Taken:

Length: 00:13:20

Location Taken: Salt Lake City, UT, US



My name is Terry Kenny and I'm a Surface-Water
Specialist with the Water Science Field Team.

And, today, we're going to talk about using
offsets in GRSAT.

So you can see I have GRSAT open on my screen

And, I am now going to load the example XML
file or primer that deals with offsets.

So, I will go File Open, click on From File.

I'm going to navigate to that XML file which
is shown right here, Offset Primer, I'm going

to select it, I'm going to click Open, and
then, I'm going to click Read Data.

So, you can see we have the measurements for
this particular site, the Sandy River near

Mercer Maine.

They plot nicely on a curve.

Typically you aren't going to see this in
the natural world too often.

But, we see this curvature in the measurements.

And, therefore, we can, likely, use an offset
to straighten out this relationship.

So, the first step you're going to take whenever
you're going to be changing offsets is you're

going to start with a new rating if you're
changing the offset.

It's the moment to change to a new rating

And so, right now, if you look in the rating
manager window over here, we're dealing with

rating number 0000.

And, I am going to clone that rating by clicking
on this button here that looks like a sheep

for cloning ratings.

And, I'm going to give this a new rating number.

I'm going to call this 1.0.

And, I'm going to leave a brief comment, new
offset developed.

Ideally, you'd leave more detailed remarks.

But, for this example, this will work.

I'm going to click Okay.

And, now you can see we are dealing with this
cloned rating.

It's shown up here rating number 1.0.

We can look in the rating period manager and
it also has a new rating that is highlighted.

You will also notice that the original rating
that we dealt with is still shown up here

in the tab.

And, I can navigate back and forth to and
from that in and of itself.

So, something you'll learn right away is GRSAT
has options in which the scales will automatically

reset themselves when you do different activities

And, you will quickly learn that, certain
actions that you take will be beneficial to

have at auto scale and certain actions that
you take will not be a benefit to have at

auto scale.

When dealing with offsets and trying to find
the proper offset for such a curve that we're

seeing here, it's typically a good idea to
have the automatic scaling of the axis on.

So, I can go up here into Setup and I'm going
to say automatic rating scale on.

And, you'll see there's a checkmark next to
it when I click on it.

And, this will insure that, when I change
the offset, the y axis will automatically

rescale itself so that you can see things

So, we're dealing with offsets.

So, we want to go into what we know is the
offset manager, click on the tab here on the

right, and we're at the point right now in
which this, these measurements are being plotted

at a 0 offset.

We can see right here 0 offset over here.

What we want to do is resolve a new offset
for this relation.

And, to do that, we're going to select offset

And so, now you'll see we have the ability
to use these radio buttons to toggle it up

or down.

But, we also want to make sure that whatever
we toggle here is reflected in the screens

or the windows shown on the left.

And so, in order to do that, we need to make
sure that we are plotting in offset 1.

So, we can see that we have a relation between
stage and discharge in this case that is concaved

when plotted at a 0 offset.

So, that tells me that I need to increase
the offset in order to get this relation to

plot on a straight.

However, before I go through the exercise
of changing the offset, I want to make sure

that I'm only dealing with measurements that
are specific to the hydraulic control that

I want to straighten out the relation for.

From my notes and the experience I have with
the station, I realize that there are two

controls represented here in this rating.

And, I'm interested in straightening out the
control or the measurements associated with

the control for streamflow below about 10,000
cubic feet per second.

And this is an exercise you're going to go
through whenever you have a station that you're

building a new rating for.

And, it's the exercise of going through and
censoring out the measurements you are not

interested in including for the resolution
of the offset or for drawing the rating curve

to it itself.

And, such censoring or filtering could be
filtering out specific control conditions

in which the control wasn't clear.

You have backwater, you have debris on the
control, that sort of thing.

You wouldn't want to include those into your
rating curve development because they bias

the way that line is being drawn.

Typically, you want to make sure that you
are developing your rating for clear control


Now, for this one, I want to filter out measurements
that are above 10,000 cubic feet per second.

So, I can go into the site visit data pane
here that I made a little bit larger, and

I can click on any of these column headings
to sort the column itself.

And, you see here, I clicked it and now I've
sorted from highest discharge down to the

lowest discharge.

And so, I want to get rid of everything above
10,000 cubic feet per second.

So, I can select this column or row right

I can push the shift button or push it down
and select the next one that I want.

And, all the other ones in between are going
to be highlighted.

I could just drag my cursor down through the
measurements I want to get rid of.

They're all highlighted here and I want to
click the X to unselect them.

Then, I'm going to go View and I'm going to
send this back into the Default Rating Shift


And, you'll see, those ones that I unselected
are now gray.

And, you can leave them gray because you're
not going to see them or not use them because

they're not colored.

Or, you can also decide to turn those off
here and you can show and hide unselected

measurements here.

And, you notice, the gray go away.

Pretty handy.

Now, like I said before, you want to make
sure that you are not biasing your rating

development to conditions on the control that
are not related to clear conditions.

And so, once again, I could take this site
visit pane out, make it a little bit larger.

I can sort by Control Conditions, by clicking

Right now all the clear ones, this is alphabetical,
are showing up first.

I'm going to click it again see if there's
any others that I'd like to get rid of.

And, what I see here, for this specific station,
is I just have some older unspecified control

conditions and clear conditions.

I don't have any here that indicate there's
debris on the control or that it's scoured

out or anything such as that that I wouldn't
want to include in this rating development.

So, at this point, I am happy with what I
filtered out, the measurements above 10,000

cubic feet per second.

And, I am ready to try to resolve a new offset
that straightens out this relation.

Once again, I'm going to go View, I'm going
to go back to the Default Rating Shift Layout,

and now, I am going to go back to the Offset
Manager here and I am going to start changing

that offset to try to get this relationship
or this relation to straighten itself out.

So, the method I follow, when I'm trying to
resolve a new offset for a curved segment

or a curved relation between stage and discharge
is I like to input two points, one near the

bottom of my curve, and one somewhere near
the top.

And, you'll see here that 2 points define
a straight line.

And, there's a straight line that goes between
these 2 points.

And, that illustrates some curvature that's
shown or seen in the relation in and of itself.

You also notice that, after I put those 2
rating points in, the variable shift diagram

becomes activated own here on the lower right
and there's some obvious curvature going on

here, when you consider that the 0 line is
represented by the rating curve that I've

just input there.

And so, when using GRSAT, this is one of the
greatest advantages is the ability to just

use the trial and error process of changing
the offset to get to a straight line relation

between stage and discharge.

So, I'm going to toggle up this offset and
see if I can get the measurements to plot

somewhere on a fairly straight line.

And so, you'll see, I went beyond the likely
offset for this segment and the rating curved

kind of curved downwards.

And now, it's not concave anymore, it's convex.

And so, at that point, I know I kind of went
past the inflexion point and I'm going to

back up this offset to where it starts to
look like those measurements are plotting

on a fairly straight line.

What I learned, from doing this little trial
and error, is that there's a good chance that

my points that I initially selected here maybe
need to be adjusted.

So, I can go to the Move Rating Point Tool
right here.

And, I believe that this point needs to come
up just a slight bit, give or take.

And, you'll notice that things start to change
in the variable shift diagram.

They also change in the rating plots, just
not quite as obvious.

And, I think I need to move this point just
a little bit as well to where I'm capturing

a little bit more of the measurements.

And what you start to see is this relationship
in the variable shift diagram that initially

had this little curve to it is starting to
straighten itself out a little bit.

And, I can toggle this offset up a little
bit more to see if I can get those to kind

of curve in.

And, you can kind of see where that inflexion
point happened.

It happened somewhere around 1.7, 1.8.

Again, this is an iterative process in which
you're going to move rating points to come

up with a place where you're going to have
this straight relation.

And then, afterwards, you're going to add
or extend your rating curve outwards one direction

or the other to capture all of the measurements.

So, I moved this lower rating point up just
a little bit.

And, I'm starting to see things straightening
out a little better over here.

I bumped it up to 1.9.

That looks decent, just to be sure, I'm going
to highlight this section over here to kind

of see what's going on.

It does suggest that there's some extra curvature
kind of going on at this lower end.

I'm going to move this up just for fun to
see if I can get things to kind of straighten

out above that point fairly well.

And, it seems like I'm doing okay here.

The fact that I can't get this entire segment
that I've decided to work with on a single

straight line suggests to me that there's
the potential for some sort of a lower water

control, hydraulic control effecting this
lower end below about 100 cubic feet per second.

It could be some sort of a section control
that's up closer towards the gauge that gets

drowned out as flow gets above say 3 feet
or so of stage to which then you're in a section

control that's defined here by this segment
that I've kind of outlined by my two rating

points here.

So what you can see is, like I said, it's
an iterative process.

You have the ability to easily role or move
or change the offset up or down by using these

little radio buttons and see how the relation
changes in respect to the straight line you

have defined with your two rating points.

So, the next step you would take here is you
would work on extending the upper and lower

portions of this rating to capture the hydraulic
control segments you're interested in and

potentially adding multiple offsets due to
the other segments that are likely to exist.

The one we just indicated or talked about
below here is likely a different hydraulic

control as is the stage and discharge values
above 10,000 cubic feet per second would be

another or third hydraulic control that you
would have to resolve an offset for.

Please check out the multiple offset video
that goes over how to deal with multiple hydraulic

controls and multiple offsets to resolve a
rating that is comprised of straight line

segments connected by smooth transitional

Thank you.